Schizophrenia is a chronic and devastating mental illness that affects approximately 1% of the world's population (Andreasen, Brain Res. Rev., 31:106 (2000)). Over 2.2 million adults age 18 years and older in the United States are diagnosed with schizophrenia, which is approximately twice the number of people stricken with Alzheimer's disease and virtually six times more than those diagnosed with insulin-dependent diabetes. Men and women are affected equally and the incident rate is nearly the same for all countries and ethnic groups throughout the world, ranking the disease among the top ten causes of disability worldwide (Andreasen, Brain Res. Rev., 31:106 (2000)).
The symptoms of schizophrenia generally appear in late adolescence or early adulthood and can be grouped into three specific categories. These include positive symptoms (auditory and visual hallucinations, disorganized thoughts and speech, delusions and irrational fears), negative symptoms (social withdrawal, adhedonia, blunted affect, lack of energy and catatonia), and cognitive dysfunction (diminished capacity for learning and memory, attention, vigilance and social cognition). Negative symptoms and especially cognitive impairment are considered to be the primary contributors to poor social functioning, the inability to work, lack of independent living skills and poor quality of life for nearly all patients (Andreasen, Brain Res. Rev., 31:106 (2000)).
The N-methyl-D-aspartate (NMDA) glutamate receptor is an ionotropic glutamate receptor involved in fast excitatory neurotransmission, which plays a key role in a variety of CNS functions, most notably long term potentiation (LTP) and neuronal plasticity. A growing body of evidence is emerging that suggests glutamatergic neurotransmission is playing a key role in the etiology of the schizophrenia. The “glutamate hypothesis” originated from the discovery that the non-competitive NMDA receptor antagonists ketamine and phencyclidine (PCP) induce positive, negative, and cognitive symptoms in healthy individuals and exacerbate symptoms in stable patients (Coyle, Cell. Mol. Neurobiol., 26:365 (2006)). These observations suggest that NMDA receptor hypofunction is playing a critical role in schizophrenia (Coyle, Cell. Mol. Neurobiol., 26:365 (2006); Millan, Psychopharmacology, 179:30 (2005)). Thus, agents that can enhance receptor function may effectively ameliorate the full symptomology of psychosis. Direct agonism is not a viable approach as it leads to excitotoxicity (M. J. Marino, et. al., J. Med. Chem., 51:1077 (2008)). Consequently, several strategies to indirectly potentiate the receptor and avoid toxic side effects are being investigated (M. J. Marino, et. al., J. Med. Chem., 51:1077 (2008); Hui, et. al., Recent Pat. CNS Drug Discov., 4:220 (2009); Stone, et. al., CNS Neurol. Diord. Drug Targets, 6:265 (2007); Gray, et. al., Mol. Psychiatry, 12:904 (2007)).
Glycine is an inhibitory neurotransmitter at strychnine-sensitive glycine receptors (GlyA site) and an excitatory neurotransmitter at the glycine modulatory site located on the NR1 subunit of the NMDA receptor (GlyB site). It is an obligatory co-agonist that allows glutamate to bind and stimulate the receptor (Harsing Jr., et. al., Current Med. Chem., 13:1017 (2006)). Thus, a method to potentiate NMDA receptor function would be to increase synaptic glycine concentrations at the GlyB site. Studies show that patients experience improvement of negative symptoms when administered glycine (0.8 g/kg/day) or D-serine (a GlyB site agonist, 0.03 g/kg/day) in conjunction with clozapine (Javitt, Biol. Psychiatry, 63:6 (2008). Large doses of these amino acids were required due to poor pharmacokinetics (PK) and CNS-penetrance; however these encouraging results provided an impetus to discover alternative approaches for increasing synaptic glycine levels.
Two high-affinity transporters that regulate synaptic glycine concentrations have been identified: GlyT-1 and GlyT-2 (Harsing Jr., et. al., Current Med. Chem., 13:1017 (2006)). Both share a high level of homology across species and approximate 50% homology with each other. Glycine transporters belong to the Na+/Cl− solute carrier 6 (SLC6) family, which includes the dopamine (DA), serotonin (5-HT), norepinephrine (NE), leucine, taurine, proline, and GABA transporters. GlyT-2 is expressed in the brainstem and spinal chord and is co-localized with strychnine-sensitive glycine receptors. GlyT-1 is primarily expressed in neuronal and glial cells of the forebrain and is largely co-localized with NMDA receptors (Harsing Jr., et. al., Current Med. Chem., 13:1017 (2006)). Thus, inhibition of GlyT-1 provides an opportunity to elevate glycine levels within close proximity to the GlyB site.
Knockout studies reveal that complete, homozygous GlyT-1 (−/−) knockout in mice is neonatally lethal, however heterozygous GlyT-1 (+/−) mice survive to adulthood and display enhanced NMDA receptor function in the hippocampus, better memory retention and no disruption in sensory gating when dosed with amphetamine (Gomeza, et. al., Handb. Exp. Pharmacols., 457 (2006)).
A small placebo controlled study was conducted whereby 20 stably treated schizophrenic patients were given sarcosine, a weak but selective GlyT-1 inhibitor, at a dose of 2 g per day. Patients exhibited improvement with negative and cognitive symptoms (Lane, et. al., Biol. Psychiatry, 63:9 (2009)).
Some glycine transporter-1 inhibitors are known in the art and have been reported to be efficacious in animal models predictive of antipsychotic activity. A handful of GlyT-1 inhibitors have advanced into Phase I, Phase II, and Phase III clinical studies. However, there is a need for other GlyT-1 inihibitors efficacious in the treatment of neurological and psychological disorders. The present invention is directed to achieving this objective.